1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor and a method for manufacturing the same. Particularly, the invention relates to a small-sized multilayer ceramic capacitor of large capacity and high reliability in which a dielectric layer is composed of barium titanate crystal grains that differ from one another in at least one selected from Ca composition concentration, Sr composition concentration and Zr composition concentration, as well as a method for manufacturing the same.
2. Description of Related Art
In recent years, as mobile equipment such as cellular phones are more widely used, and as semiconductor devices that are the main components of personal computers have higher speed and higher frequency, multilayer ceramic capacitors for these electronic equipment are increasingly required to have smaller size and larger capacity.
For this reason, in a dielectric layer constituting a multilayer ceramic capacitor, attempts are made to form it in a smaller thickness and a higher number of laminations. For example, Japanese Patent Application Laid-Open No. 2003-40671 employs, as dielectric powder constituting dielectric ceramic, a mixture of barium titanate powder in which part of A site is substituted by Ca (hereinafter referred to as “BCT powder”) and barium titanate power containing no substitution Ca (hereinafter referred to as “BT powder”). This improves atomicization of barium titanate crystal grains, relative dielectric constant, and DC bias characteristic in a dielectric layer after firing.
When the BCT crystal grain in the barium titanate crystal grains disclosed in Japanese Patent Application Publication Laid-Open No. 2003-40671 is fired after mixing with additives such as Mg, a rare earth element, etc. essential in controlling the temperature characteristic of relative dielectric constant, grain growth is apt to occur as Ca contained in the BCT powder is diffused, which necessitates the rigid conditional control during firing. Especially, the use of raw material whose particle diameter is not more than sub-micron may cause significant grain growth. It is therefore known that it is not easy to manufacture a sintered body composed of fine-grain barium titanate crystal grains.
For this reason, Japanese Patent Application Publication Laid-Open No. 2003-40671 further adds MnCO3, MgO, and an oxide of a rare earth element, when BT powder covered with an oxide of Mg and a rare earth element is mixed with BCT powder, in order to suppress the grain growth of BCT crystal grain during firing. It is described that, by so doing, a coating layer composed of a composite oxide of high insulating property can be formed approximately uniformly on the surface of the BT crystal grain after firing, and the excess solid solution and grain growth of Mg and a rare earth element with respect to the BCT crystal grain can be suppressed.
In accordance with the manufacturing method described in the above Publication, when employed firing conditions under which there is used a small-sized experimental kiln permitting high control of firing temperatures, a sample satisfying desired relative dielectric constant, temperature characteristic and high-temperature load test characteristic can be formed even with the technique of adding MnCO3, MgO, and an oxide of a rare earth element when the above-mentioned BT powder covered with an oxide of Mg and a rare earth element is mixed with the BCT powder.
On the other hand, in the management level of firing temperature with regard to such a large-sized tunnel type kiln as used for high-volume manufacturing of multilayer ceramic capacitors, there is large variations in maximum temperature during firing within the kiln. Therefore, the grain growth of BCT crystal grain is apt to cause variations during high-volume manufacturing, so that there occurs many poor in relative dielectric constant, temperature characteristic, and high-temperature load test characteristic, thereby lowering the yield.
Recently, as other high dielectric constant material for multilayer ceramic capacitor, barium titanate (Ba1-xSrx)TiO3 powder in which Ca site of the above-mentioned BCT powder is substituted by Sr (hereinafter referred to as “BST powder”) has been found and used in a dielectric layer for large-capacity multilayer ceramic capacitor and a thin film capacitor (see Japanese Patent Application Publication Laid-Open No. 2004-262717 and No. 2004-281446).
As a high dielectric constant material for multilayer ceramic capacitor, (Ba1-xCax)m(Ti1-yZry)O3 (hereinafter referred to as “BCTZ”) in which part of Ti site of the above-mentioned BCT powder is substituted by Zr (see Japanese Patent Application Publication Laid-Open No. 11-157928), and (Ba1-x-yCaxSry)mTiO3 (hereinafter referred to as “BCST”) in which part of Ba site together with Ca of the above-mentioned BCT powder is substituted by Sr (see Japanese Patent Application Publication Laid-Open No. 2002-284571), have also been found as a high dielectric constant material, and used in a dielectric layer for high-capacity type multilayer ceramic capacitor and a thin film capacitor.
However, even with such a high dielectric constant material, the same problem as described above is occurring in high-volume manufacturing.